Exhaust purification device of an internal combustion engine

Description

TECHNICAL FIELD

The present invention relates to an exhaust purification device of an internal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine arranging in the engine exhaust passage an NO_X storage catalyst which stores NO_X contained in the exhaust gas when the air fuel ratio of the inflowing exhaust gas is lean and releases the stored NO_X when the air fuel ratio of the inflowing exhaust gas becomes a stoichiometric air fuel ratio or rich and arranging in the engine exhaust passage upstream of this NO_X storage catalyst a compact three-way catalyst (see for example Japanese Patent Publication (A) No. 2004-108176). In this internal combustion engine, if the NO_X storage ability of the NO_X storage catalyst approaches saturation, the air fuel ratio of the exhaust gas is temporarily made rich whereby NO_X is released from the NO_X storage catalyst and reduced.

However, there is a problem in that, in this internal combustion engine, when making the NO_X storage catalyst release NO_X by feeding mist fuel upstream of the NO_X storage catalyst, the NO_X released from the NO_X storage catalyst cannot be properly reduced.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purification device of an internal combustion engine able to reduce the NO_X released from an NO_X storage catalyst well.

According to the present invention, there is provided an exhaust purification device of an internal combustion engine arranging an NO_X selective reducing catalyst in an engine exhaust passage, arranging an NO_X storage catalyst able to store NO_X contained in the exhaust gas in the engine exhaust passage upstream of the NO_X selective reducing catalyst, arranging a fuel feed valve in the engine exhaust passage upstream of the NO_X storage catalyst to feed a mist fuel from the fuel feed valve to the NO_X storage catalyst, reacting NO_X stored in the NO_X storage catalyst and fed fuel on the NO_X storage catalyst to produce an intermediate product comprising bonded molecules of NH and a hydrocarbon molecule more than an equivalent ratio with respect to one NO_X molecule and making the intermediate product produced in the NO_X storage catalyst be adsorbed at the NO_X selective reducing catalyst so as to reduce NO_X in the exhaust gas by the adsorbed intermediate product.

That is, in the present invention, by feeding mist fuel from the fuel feed valve to make the NO_X storage catalyst release NO_X and reducing the released NO_X to, NH₂, the NO_X is purified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of a compression ignition type internal combustion engine, and FIG. 2 is a cross-sectional view of the surface part of a catalyst carrier of an NO_X storage catalyst.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is an overview of a compression ignition type internal combustion engine.

Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electronically controlled fuel injector injecting fuel into each combustion chamber 2, 4 an intake manifold, and 5 an exhaust manifold. The intake manifold 4 is connected through an intake duct 6 to a compressor 7a of an exhaust turbocharger 7, while the inlet of the compressor 7a is connected through an intake air amount detector 8 to an air cleaner 9. Inside the intake duct 6, a throttle valve 10 driven by the step motor is arranged. Further, around the intake duct 6, a cooling device 11 for cooling the intake air flowing through the intake duct 6 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is led into the cooling device 11 where the engine cooling water is used to cool the intake air.

On the other hand, the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7b of the exhaust turbocharger 7. The outlet of the exhaust turbine 7b is connected to the inlet of a NO_X storage catalyst 12 able to store the NO_X contained in the exhaust gas. The cutlet of the NO_X storage catalyst 12 is connected to a NO_X selective reducing catalyst 14 via an exhaust pipe 13. Further, a fuel feed valve 15 for feeding a fuel into the exhaust gas flowing within the exhaust manifold 5 is attached to the exhaust manifold 5.

The exhaust manifold 5 and intake manifold 4 are connected to each other through an exhaust gas recirculation (hereinafter referred to as “EGR”) passage 16. Inside the EGR passage 16, an electronic control type EGR control valve 17 is arranged. Further, around the EGR passage 16, a cooling device 18 for cooling the EGR gas flowing through the EGR passage 16 is arranged. In the embodiment shown in FIG. 1, engine cooling water is led to the cooling device 18 where the engine cooling water cools the EGR gas. On the other hand, each fuel injector 3 is connected through a fuel tube 19 to a common rail 20. This common rail 20 is fed with fuel from an electronically controlled variable discharge fuel pump 21. The fuel fed into the common rail 20 is fed through each fuel tube 19 into the fuel injectors 3.

Initially, the NO_X storage catalyst 12 will be explained. This NO_X storage catalyst 12 is comprised of a substrate on which for example a catalyst carrier comprised of alumina is carried. FIG. 2(A), (B) illustrates the cross-section of the surface part of this catalyst carrier 30. As shown in FIG. 2(A), (B), the catalyst carrier 30 carries a precious metal catalyst 31 diffused on the surface. Further, the catalyst carrier 30 is formed with a layer of an NO_X absorbent 32 on its surface.

In the embodiment according to the present invention, as the precious metal catalyst 31, platinum Pt is used. As the ingredient forming the NO_X absorbent 32, for example, at least one element selected from potassium K, sodium Na, cesium Cs, and other such alkali metals, barium Ba, calcium Ca, and other such alkali earths, lanthanum La, yttrium Y, and other rare earths is used.

If the ratio of the air and fuel (hydrocarbons) fed into the engine intake passage, combustion chamber 2, and exhaust passage upstream of the NO_X storage catalyst 12 is called the “air-fuel ratio of the exhaust gas”, an NO_X absorption and release action such that the NO_X absorbent 32 absorbs the NO_X when the air-fuel ratio of the exhaust gas is lean and releases the absorbed NO_X when the oxygen concentration in the exhaust gas falls is performed.

That is, explaining this taking as an example the case of using barium Ba as the ingredient forming the NO_X absorbent 32, when the air-fuel ratio of the exhaust gas is lean, that is, the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas, as shown in FIG. 2(A), is oxidized on the platinum Pt 31 to become NO₂, next is absorbed in the NO_X absorbent 32 and bonds with the barium oxide BaO to diffuse in the form of nitrate ions NO₃⁻into the NO_X absorbent 32. In this way, NO_X is absorbed in the NO_X absorbent 32. So long as the oxygen concentration in the exhaust gas is high, NO₂ is formed on the platinum Pt 31. So long as the NO_X absorbent 32 is not saturated in NO_X absorption ability, NO₂ is absorbed in the NO_X absorbent 32 and nitrate ions NO₃⁻are formed.

As opposed to this, for example if the exhaust gas is made a rich air-fuel ratio or stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the reverse direction (NO₃⁻→NO₂), therefore the nitrate ions NO₃⁻in the NO_X absorbent 32 are released in the form of NO₂ from the NO_X absorbent 32.

On the other hand, the NO_X selective reducing catalyst 14 is comprised of an ammonia adsorption type Fe zeolite or a titania/vanadium based-catalyst having no ammonia adsorption function, which are capable of selectively reducing the NO_X in the exhaust gas by ammonia when the air-fuel ratio of the exhaust gas is lean. In the embodiment shown in FIG. 1, the NO_X selective reducing catalyst 14 is comprised of an ammonia adsorption type Fe zeolite.

Now, when the air-fuel ratio of the exhaust gas is lean as mentioned above, that is, when combustion is performed under a lean air-fuel ratio, the NO_X in the exhaust gas is absorbed in the NO_X absorbent 32. However, if combustion is continued under a lean air-fuel ratio, the NO_X absorption ability of the NO_X absorbent 32 will end up becoming saturated and therefore the NO_X absorbent 32 will end up unable to absorb NO_X . Here, in an embodiment of the present invention, before the absorption ability of the NO_X absorbent 32 becomes saturated, fuel is fed from the fuel feed valve 15 to make the NO_X storage catalyst 12 release NO_X. This will be explained in the following.

In this embodiment of the present invention, diesel fuel or a heavy fuel having diesel fuel as a main ingredient is fed in a mist state, that is, in the form of particulates, from the fuel feed valve 15. Part of the fed fuel is oxidized, but the majority, as shown in FIG. 2(B), adheres to the surface of platinum Pt 31 and the surface of the NO_X absorbent 32. If the fed fuel adheres to the surface of the platinum Pt 31, the oxygen concentration on the surface of the platinum Pt 31 will fall, causing the NO₃⁻of the NO_X absorbent 32, as shown in FIG. 2(B), to be released in the form of NO₂.

If a large amount of fuel of an extent whereby the air-fuel ratio of the exhaust gas becomes considerably rich is fed from the fuel feed valve 15, that is, if the reducing agent for reducing the NO_X is fed in a large amount, the released NO₂, as shown in FIG. 2(B), will be reduced to NO and then to NH₂. Next, this NH₂ immediately reacts with the hydrocarbons HC adhering to the platinum 31, whereby, as shown in FIG. 2(B), an intermediate product 33 comprising the bonded molecules of the hydrocarbons HC and NH₂ is produced. Note that the number of carbon atoms of the hydrocarbons HC in the fed fuel is considerably large, accordingly, in the NO_X storage catalyst 12, the stored NO_X and the fed fuel produce an intermediate product comprising bonded molecules of NH₂ and a hydrocarbon molecule more than an equivalent ratio with respect to one NO_X molecule.

Thereby, the NO_X stored in the NO_X storage catalyst 12 is released from the NO_X storage catalyst 12 by the fed fuel, and the released NO_X is reduced.

Next, the intermediate product produced in the NO_X storage catalyst 12 is fed into the NO_X selective reducing catalyst 14 and is adsorbed at the NO_X selective reducing catalyst 14. The intermediate product adsorbed at the NO_X selective reducing catalyst 14 is broken down to hydrocarbons HC and ammonia NH₃ in the NO_X selective reducing catalyst 14 if the temperature of the NO_X selective reducing catalyst 14 rises. The hydrocarbons HC are oxidized by the oxygen contained in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, whereby the NO_X contained in the exhaust gas is reduced by the ammonia NH₃ adsorbed at the NO_X selective reducing catalyst 14.

In this way, the NO_X stored in the NO_X storage catalyst 12 is transferred in the form of amine NH₂ to the NO_X selective reducing catalyst 14, converted to ammonia NH₃ in the NO_X selective reducing catalyst 14, and used for the purification of NO_X.

LIST OF REFERENCES

• 4 . . . intake manifold
• 5 . . . exhaust manifold
• 7 . . . exhaust turbocharger
• 12 . . . NO_X storage catalyst
• 14 . . . NO_X selective reducing catalyst
• 15 . . . fuel feed valve